1. Field of the Invention
The present invention relates generally to the manufacture of melt-spun polymeric filaments. More particularly, it relates to the manufacture of polyester filaments by a process involving the use of an improved spinnerette. The improved spinnerette has groups of orifices with specifically defined unequal dimensions from group to group, rather than similar dimensions. It also relates to the improved filamentary product thereby obtained, particularly at high extrusion rates of molten polymer through the pack containing the spinnerette.
2. Prior Art
The manufacture of melt-spun polymeric filaments is extremely old in the art. Typically, a molten polymer (such as polyester, polyamide and polyolefin) is extruded downwardly through a plurality of orifices in the spinnerette to form molten filaments. The extruded filaments are simultaneously cooled in a quench zone and stretched (by yarn haul-off means such as a yarn winder) into finer filaments having at least some molecular orientation (expressed as birefringence, .DELTA.n).
High variability of molecular orientation of the melt-spun filaments is also well-known to affect deleteriously downstream processes and/or the properties of downstream products made therefrom, such as drawn yarns. It is also well known that high productivity processes (e.g., involving the extrusion of several hundred pounds of molten polymer each hour through a single spinnerette) tend to result in the production of filaments having higher birefringence variability than filaments made at lower extrusion rates. There is thus a problem in maintaining the quality of the melt-spun yarn when production rates are increased. U.S. Pat. No. 4,332,764 (Brayford and Cardell) discloses one method of reducing birefringence variability in polyester filaments melt-spun at several hundred pounds per hour.
All prior art relating to the melt-spinning of polyester polymer into filaments has apparently involved the use of spinnerettes in which the corresponding dimensions of the individual orifices within the spinnerette have been essentially equal within machinable tolerance limits. This is perhaps not surprising since (i) high birefringence variability is often associated with high denier variability; and (ii) variations between orifices causes denier variability.
Two major classes of prior art relative to the invention claimed hereinafter are discussed below. The first class of prior art relates primarily to theories and mathematical models that have been advanced. The second class of prior art relates primarily to concrete experimental data from the patent literature.
Within the first class of prior art, many attempts have been made to understand the science of melt-spinning polyester polymer. One recent comprehensive publication in the subject area is "Model of Steady-State Melt Spinning at Intermediate Take-Up Speeds" by Dr. H. H. George, published in April 1982 by "Polymer Engineering and Science". Dr. George also gave an oral presentation in Hawaii in 1979 on a related topic. Another publication of interest is "Fundamentals of Fibre Formation" by Andrzej Ziabicki, published by John Wiley & Sons in 1976. Pages 149-248 relate to melt-spinning. An older publication of interest is "Studies on Melt-Spinning. II. Steady State and Transient Solutions of Fundamental Equations Compared with Experimental Results" by Susumu Kase and Tatsuki Matsuo, found in the "Journal of Applied Polymer Science", Volume 11 at pages 251-287 (1967). While all the foregoing publications are valuable contributions to developing a qualitative understanding of the science of melt-spinning polyester polymer, it is believed not unfair to state they still fall far short of enabling a research worker to predict how to further reduce birefringence variability in high productivity processes such as that disclosed in the Brayford and Cardell patent. This is so for reasons including the following. Firstly, all the models are based upon a large number of simplifying assumptions. Secondly, a very large number of interdependent variables are involved in the various mathematical formulae and, as a result, the models tend to have value in predicting qualitative trends under single filament steady state conditions, rather than quantitative trends under multifilament transient conditions. Nonetheless, at least two aspects of the theories developed in the foregoing publications are at least of interest to the instant invention. In particular, firstly, it is well known that increasing the diameter of a circular orifice in a melt-spinning process involving the extrusion of a single filament, without introducing any other changes to independent variables, thereby decreases the extrusion velocity of the filament from the spinnerette; reduces the pressure drop across the orifice; reduces the extrusion temperature; has no effect on take-up speed or take-up denier; increases the final tension of the filament at take-up; and increases the final birefringence of the filament. Secondly, some models suggest that there is a correlation at each and every point in the spinning threadline between the birefringence and the stress at the same point, expressed in grams per denier. Further, George suggests that the foregoing correlation is, in fact, unique. In which case, George's equations lend themselves to predicting what compensatory changes might be made in a pair of groups of filaments when the first group of filaments is subjected to different quench conditions from the second group of filaments. Nevertheless, the fact remains, that the prior art does not show any extrusion of molten polyester polymer through a spinnerette having orifices of differing dimensions within the single spinnerette. Further, the equations of the published prior art cannot be used to accurately predict the actual changes in filament denier that occur as a result of so-doing. Even less, therefore, can they be used to predict the resultant compensatory effects in birefringence. In addition, the prior art teaches that high transverse air quench rates across a single filament result in the filament having asymmetric birefringence across the filament in the direction of quench gas flow. There is, inevitably, a tendency for asymmetric birefringence to occur at the very high cross flow quench rates required when spinning molten polyester at very high throughputs. Accordingly, the foregoing models are, at best, believed to be a guide post concerning the nature of experiments that might perhaps be performed in order to reduce the birefringence variability of polyester melt-spun filaments.
Within the second class of prior art defined above, several patents discussed below are, at least, of interest to the present invention.
Firstly, U.S. Pat. No. 4,248,581 (Harrison) also addresses the problem of obtaining filaments with uniform physical properties in high throughput, high filament density melt-spinning processes. The patent points out that the prior art recognizes that uniform, turbulence-free quenching of filaments is an important factor in the production of filaments having uniform physical properties, a prerequisite to acceptable performance of fibers in subsequent processes. It also points out that this is difficult to achieve in the cross-flow quench system, typically linked to a high throughput and high filament density melt-spinning process, as the traverse path of the quenching fluid causes it to contact first one side of the filament bundle and then pass therethrough. Those filaments most remote (downstream) from the entry of the quench fluid are cooled or solidified by a quench flow which has been pre-heated, made more turbulent and substantially diminished (via a downward moving boundary layer) by the obstruction presented by filaments closer to and previously contacted by the quench fluid. As a consequence, the cooling rate of the filaments is progressively slower as quench fluid passes through the filament bundle. The patent further points out that the ideal solution to quench irregularity would be to increase the spacing of spinnerette orifices, resulting in increased distance between filaments for quenching. However, there are practical restraints to the increase in orifice spacing in a spinnerette of given diameter and orifice count. The patent then points out that the prior art has attempted to solve quench irregularity by rearranging the positions of the spinnerette orifices within the spinnerette plate. For example, it discusses the use of "V" patterns, concentric circles, crescent formations, rectangular grids, and irregular arrangements whereby the spinnerette orifices are staggered so that each one is located in the quench flow path without obstruction. It also discusses the use of spinnerette orifices arranged in parallel rows, such that the orifices in a given row are equally spaced and the distance between adjacent rows is less than the distance between the orifices in each row. The invention disclosed in the '581 patent also relates to a spinnerette in which the orifices are arranged in a specific configuration. Nowhere does the patent remotely suggest the possibility of varying the dimensions from orifice to orifice within the spinnerette in order to improve the uniformity of the final product.
Secondly, U.S. Pat. No. 4,104,015 (Meyer) also addresses the problem of filament non-uniformity. In particular, the patent points out (at column 1, beginning at line 23) that one of the most significant factors contributing to filament non-uniformity during the melt-spinning process is the fact that the temperature of the molten polymer passing through the orifices positioned near the center of the spinnerette is higher as compared to the temperature of the molten polymer passing through the orifices positioned near the edge of the spinnerette. The higher the temperature of the polymer, the lower the viscosity; and the lower the viscosity the faster the polymer under a given pressure passes through an orifice of the spinnerette. Therefore, because of the temperature differential across the face of the spinnerette, the flow rate of the molten polymer through the orifices of the spinnerette varies, and this results in filament (denier) non-uniformity. Although attempts have been made to reduce the temperature differential across the face of the spinnerette and thus improve the uniformity of the filaments, non-uniformity is still a problem. The invention of the '015 patent essentially amounts to the use of an improved bridge plate in which the position of the orifices are adjusted to adjust the pressure above each spinnerette orifice. Thereby the temperature non-uniformity is compensated. It should also be noted that Applicants' assignee commercially used in secret in the 1960's a process involving a somewhat different solution. In particular, in the spinning of nylon 6,6 polymer, observed temperature differentials across the face of the spinnerette were in part compensated for by enlarging the orifices in the cooler portion of the spinnerette. The inventors of the instant invention were unaware of that old work at the time that they conceived their invention and initially reduced it to practice. Further, it should be noted that the work on nylon 6,6 involved enlarging the orifices remote from the quench source (in contrast to the instant invention that is described hereinafter). Further, it should be noted that the work on nylon 6,6 involved the production of continuous filament yarn from relatively small packs at relatively low polymer throughputs per square inch of spinnerette face (in contrast to the invention described hereinafter in which high polymer throughputs per square inch of spinnerette face are used).
Thirdly, U.S. Pat. No. 2,766,479 (Henning) is of interest in that FIG. 3 discloses a plate having orifices of different size therein. The patent relates to the extrusion of cellular plastics upon filamentary conductors. It is pointed out that in order to prevent premature gas expansion within the confines of the extruder, it is important that the temperatures within the extruder and the dye should be accurately regulated, and that the rate of extrusion and the linear speed of the conductive core be adjusted suitably. This may be accomplished by creating a back pressure within the extruder to prevent premature expansion of the gas therein. The plate shown in Henning's FIG. 3 merely relates to such a plate that creates back pressure against the extruder screw and is positioned upstream of the extrusion dye.
Fourthly, U.S. Pat. No. 3,628,930 (Harris) also discloses a baffle plate upstream of the spinnerette, apparently in order to control melt pressure above the spinnerette orifices, which spinnerette orifices appear to be of uniform size.
Fifthly, U.S. Pat. No. 2,030,972 (Dreyfus) discloses in FIG. 2 a spinnerette which at first sight might appear to have larger orifices 16 in the outer ring than orifices 17 in the inner ring. The text of the patent, however, does not confirm this. Indeed, it is pointed out "the size of the orifices is much exaggerated" (page 2, column 2, lines 5-6).
Sixthly, U.S. Pat. No. 3,457,342 (Parr et al) discloses a plate upstream of a spinnerette in which the orifices 15 are smaller in size than the orifices 14 (see FIGS. 2 and 3, in particular). However, the extrusion orifices 3 all appear to have similar dimensions.
Seventhly, U.S. Pat. No. 3,375,548 (Kido et al) discloses in FIG. 1 a pack for producing conjugated filaments in which the spinning orifices 14 are fed with polymer from two other upstream orifices 21 and 22, which orifices 21 and 22 apparently may differ in size. However, there appears to be no suggestion that spinnerette orifices 14 should have different dimensions from each other.
Eighthly, several U.S. patents originally thought to be of interest are believed to be less pertinent than the aforementioned prior art. They are U.S. Pat. Nos. 4,123,208 (Klaver et al); 3,867,082 (Lambertus et al); and 3,311,688 (Schuller).
Ninthly, some patents relate to filamentary products deliberately made with mixed filament deniers. For example, U.S. Pat. No. 3,965,664 (Goetti et al) relates to a spun yarn made from a mix of staple fibers, in which the mix is formed from staple fibers of at least three different titers. The patent further teaches generally that the synthetic plastic fibers may, for instance, be of the type extruded from orifices of different size or different cross-section (at column 3, lines 17-19). There is, however, no specific exemplification thereof. Even less is there any recognition of criticality concerning the location of the larger orifices relative to the location of the smaller orifices.
Tenthly, Russian Pat. No. 419,485 is understood to disclose that the packing density of glass fibers is increased by having a mix of widely different deniers; and that such a product can be made by using a spinnerette having a mixture of orifice sizes. However, glass is not a polymeric orientatable material.
In sum, nowhere does the prior art disclose or suggest the invention claimed hereinafter.